1. Field of the Invention
This invention relates to vertical cavity surface emitting lasers (VCSELs). More specifically, it relates to tunnel junctions for long-wavelength VCSELS.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics.
VCSELs include semiconductor active regions, which can be fabricated from a wide range of material systems, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Some VCSELs, particularly those used at long-wavelengths, incorporate tunnel junctions. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD).
FIG. 1 illustrates a typical long-wavelength VOSEL 10 having a tunnel junction. As shown, an n-doped InP substrate 12 has an n-typo electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the InP substrate 12, and an n-type graded-index InP lower spacer 18 is disposed over the lower mirror stack 16. An InGaAsP or AlInGaAs active region 20, usually having a number of quantum wells, is formed over the InP lower spacer 18. Over the active region 20 is a tunnel junction 25. Over the tunnel junction 25 is an n-type graded-index InP top spacer 22 and an n-type InP top mirror stack 24 (another DBR), which is disposed over the InP top spacer 22. Over the top mirror stack 24 is an n-type conduction layer 9, an n-type cap layer 8, and an n-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that provides current confinement. The insulating region 40 is usually formed either by implanting protons into the top mirror stack 24 or by forming an oxide layer. In any event, the insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path though the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the electrical contact 26 toward the electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that the current flows through the conductive central opening 42 and into the tunnel junction 25. The tunnel junction converts incoming electrons into holes that are injected into the active region 20. Some of the injected holes are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the conduction layer 9, through the cap layer 8, through an aperture 30 in electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical long wavelength VCSEL having a tunnel junction, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate), different material systems can be used, operational details can be tuned for maximum performance, and additional structures and features can be added.
While generally successful, VCSELs similar to that illustrated in FIG. 1 have problems. One problem in realizing commercial quality long wavelength VCSELs is the available mirror materials. Since long wavelength VCSELs are often based on InP, for proper lattice matching InP/InGaAsP or AlInAs/AlInGaAs mirrors are often used. However, because those materials have relatively low refractive index contrasts, 40-50 mirror pairs are typically needed to achieve the required high reflectivity. Growing that number of mirror pairs takes a long time, which increases the production costs.
Another problem, which is addressed by the tunnel junction 25, is optical loss. In long wavelength VCSELs it is often critical to limit optical losses. To that end, p-doped materials, which absorb more light than n-doped materials, are replaced by n-doped materials and the tunnel junction 25. That junction converts holes into electrons that are injected into the active region.
Tunnel junctions used in semiconductor lasers are thin (say 10 nanometer), reversed biased structures. Such tunnel junctions are usually n++/p++ structures in which the p-region is highly doping (greater than 1×1019 cm−3) using a low diffusivity dopant such as carbon. This enables a low voltage drop in a device having low free carrier absorption and sufficient free carriers.
Prior art semiconductor laser tunnel junctions have been reported using MBE-grown Be-doped InGaAsP or MOCVD grown C-doped AlAs. However, the reported doping in InGaAsP appears insufficient, while the strain of AlAs on InP materials appears excessive. Thus, a new long wavelength VCSEL would be beneficial. Even more beneficial would be a new tunnel junction suitable for use in long wavelength VCSELs. Still more beneficial would be new tunnel junctions that use MOCVD-grown layers and that are suitable for use in long wavelength VCSELs.